The establishment of the cardiovascular system represents an early, critical event essential for normal embryonic development. An important component of vascular ontogeny is the differentiation and development of the endothelial and endocardial cells and subsequent organization into vascular channels. This involves, at least in part, the expression and regulation of specific cell surface receptors required to mediate cell-cell and cell-matrix adhesion. Our previous work has documented that Platelet Endothelial Cell Adhesion Molecule (PECAM-1), a novel member of the immunoglobulin superfamily, is one of the earliest detectable receptors uniquely expressed on the endothelium and endocardium and it is present as functionally distinct, alternatively spliced isoforms during development. The purpose of this proposal is to determine the role of PECAM-1 in vasculogenesis and early cardiac development. We propose to disrupt the program of PECAM-1 expression in the whole mouse embryo culture system using: a) adenoviral mediated gene transduction to overexpress PECAM-1 in endothelial cells, b) PECAM-1 specific function blocking antibodies to perturb adhesion, and c) adenoviral mediated antisense RNA constructs to attenuate PECAM-1 expression. We will also characterize the developmental expression of alternatively spliced isoforms through a) production of isoform specific antibodies and immunohistochemical localization of temporal and spatial expression of isoform variants and b) administration of isoform specific antibodies to determine abnormalities in vascular morphogenesis that occur in a unique in vitro model of vasculogenesis. In addition, we will characterize the 5' regulatory regions of the PECAM-1 gene that control endothelial specific expression. Finally, we will delineate alterations in the development of the extraembryonic and embryonic circulation that occur as a result of a null mutation in the PECAM-1 allele. We will a) "knock in" cre recombinase into the PECAM-1 locus to produce simultaneous inactivation of the allele and endothelial specific expression of cre and b) use cre-loxP site specific recombination to produce a conditional PECAM-1 null mutation. These studies will provide a complementary and comprehensive analysis of PECAM-1 mediated cell adhesion during vasculogenesis and will also provide models useful for the study of other mechanisms important for normal vascular ontogeny.